Like I had a sexual experience with another girl and I got like erect and shewas like on top. I'm curious is it normal to have this happen. I could actually feel inside her vagina. She kept acting like it was different. And just curious. Like if a girl's clitoris gets enlarged. Like is that normal? Also how small is a clitoris supposed to be?

Yes, girl can penetrate another girl.
Idk if you can actually penetrate inside her vagina with your clit, but if you have big and firm clit you can place it inside her lips and that would probably feel like being "inside" her. At least my BF says so when I rub myself with tip of his penis, he says he feels like he's inside me (but he isn't).

Yeah like ivaiva said, technically there could be penetration with a big enough clitoris, but other than that, you'd need something like a strap on, or dildo to penetrate another woman as a female (sans penis) as yourself.